There has recently been a strong demand for a miniaturized high-resolution imaging apparatus as an imaging apparatus mounted on a mobile phone or a personal computer (PC). In response to the demand, the cell pitch of an image sensor such as a charged couple device (CCD) or complementary metal oxide semiconductor (CMOS) image sensor has been dramatically reduced, so that an optical system is demanded to provide higher image-forming performance with less optical aberrations than before.
To satisfy the demand, the number of lenses has been increased. For example, there has been proposed a configuration including five aspherical plastic lenses.
However, increasing the number of lenses will reduce productivity since the increased number of lenses necessitate high assembly accuracy and because of the fundamental physical properties, such as minute birefringence, of plastic lenses themselves.
Accordingly, there has been proposed technology to simplify a lens by spherically curving an image sensor to reduce necessity of image plane correction (see Patent Literature 1, for example).
Patent Literature 1 describes a front-diaphragm optical system having a configuration of two lenses in one group, in which separately molded two plastic lenses are bonded together. However the technology in Patent Literature 1 has problems such as the optical system including no infrared cut-off filter, and the curvature of the image sensor causing some optical barrel distortion, and thus has been applicable only to a low-pixel image such as an image in common intermediate format (CIF) or video graphics array (VGA) format.